As a method for recording digital data in an optical disk medium, there is often employed a method in which the linear velocity is kept constant so that a recording density on the recording medium is made uniform, as adopted for a compact disk (hereinafter, abbreviated as a “CD”) or a DVD (Digital Versatile Disk). When digital data is reproduced with respect to an optical disk reproduction signal which is subjected to mark width modulation, and digitally modulated and recorded so that a linear recording density is constant, a phase of a clock component corresponding to a channel bit frequency of the reproduction signal is detected, to construct a phase locked loop, thereby performing phase synchronous pull-in.
At this time, when the frequency of the clock component of the reproduction signal is vastly different from a frequency of a clock which is generated by the phase locked loop, it is highly likely that the phase synchronous pull-in is not completed or at pseudo pull-in to a frequency that is different from a frequency to be pulled in is performed. To avoid these problems, a reproduction linear velocity cycle is detected from a specific pulse length or pulse interval included in the reproduction signal, and a disk rotational speed or free-running frequency of the phase locked loop is controlled on the basis of the detected reproduction linear velocity cycle, whereby normal phase synchronous pull-in can be performed.
Conventionally, for example, there is a disk reproduction system as shown in FIG. 27, that enables normal phase synchronous pull-in. In this conventional optical disk reproducing device, digital recording codes as shown in FIG. 28(a) are recorded on an optical recording medium 50 such as an optical disk so as to have a constant linear recording density. It is assumed, for example, that recorded data is data in which the number of continuous pieces of data “0” or data “1” is restricted to be 3 to 14 as in an 8-16 modulation system. As shown in FIG. 28(a), a signal obtained by reproduction by means of a reproduction means 51, such as an optical pickup, has an amplitude attenuated due to greater interference for a higher frequency component, with an increase in a recording density of the recorded data in the linear direction. Therefore, this signal is amplified by a not-shown preamplifier and, thereafter, corrected so that a high frequency component is emphasized, by a waveform equalization means 2.
The high-frequency-emphasized reproduction signal as shown in FIG. 28(b) is sampled into a multi-bit digital signal by an analog/digital converter 3, which is a means for converting an analog signal into a digital signal by employing a reproduction clock generated by a VCO (Voltage Controlled Oscillator) 62. At this time, when a phase of the reproduction clock is synchronized with a phase of a clock component of the reproduction signal, sampled data as shown in FIG. 28(c) is obtained. FIG. 28(c) shows sampled data which is suited particularly for a Partial Response Maximum Likelihood (hereinafter, abbreviated as a “PRML”) signal process system.
The PRML signal process system is one which applies a partial response system in a reproduction system in which an amplitude of a high frequency component is deteriorated with an increase in a recording density in the linear recording direction, and thus a signal-noise ratio is increased, and intentionally adds waveform interference to the reproduction system, so as to realize a reproduction system which requires no high frequency component, as well as increases the quality of reproduction data by a maximum likelihood decoding method for estimating the most likely series by probability calculation in consideration of the waveform interference. The increase in the recording density in the linear recording direction is adopted as one of the methods for increasing the recording density when, for example, a recording capacity is increased from a CD to a DVD.
This sampled multi-bit digital signal is inputted to an offset correction means 52, thereby correcting an offset component included in a reproduction digital signal. The reproduction digital signal which is offset-collected by the offset correction means 52 is subjected to partial response equalization by a transversal filter 53. At this time, by applying partial response equalization, a multi-valued equalized output signal is obtained as shown in FIG. 28(d). A weight coefficient of a tap of the transversal filter 53 is supplied from a tap weight coefficient setting means 54 by employing an LMS (Least Mean Square; hereinafter, abbreviated as “LMS”) algorithm for minimizing a square mean value of an equalization error. An output signal from the transversal filter 53 is demodulated into binarized digital data by a viterbi decoder 55 as a kind of a maximum likelihood decoder.
A phase synchronous reproduction clock when sampling is performed by the analog/digital converter 3 is controlled as follows.
Initially, a position of crossing the zero level is continuously detected from an output signal from the offset correction means 52, a synchronous pattern length in a specific period of one or more frames is detected by employing output from a zero-crossing length detector 56 which counts the number of sampling between neighboring zero-crossing positions, and a frequency error amount for controlling frequency of the reproduction clock is decided by a frequency error detector 57 which detects a cycle of detecting a synchronous pattern.
Phase information of reproduction digital data is detected by a phase comparator 58 by employing the output signal from the offset correction means 52, and a phase error amount for controlling phase synchronization between the reproduction clock and the reproduction digital data is decided. An output signal from a frequency control loop filter 59 is converted into an analog signal by a digital/analog converter 61b, so that the frequency is controlled by employing the frequency error amount outputted from the frequency error detector 57 until an area where the reproduction clock can be synchronized with the reproduction digital signal is reached, and a VCO 62 is controller by the output signal from the digital/analog converter 61b. On the other hand, an output signal from a phase control loop filter 60 is converted into an analog signal by a digital/analog converter 61a, so that the reproduction clock is synchronized with the reproduction digital signal by employing the phase error amount outputted from the phase comparator 58, and the VCO 62 is controlled by the output signal from the digital/analog converter 61a. Actually, in this prior art example in FIG. 27, the output signal from the digital/analog converter 61b and that from the digital/analog converter 61a are added together by an adder 63, an the VCO 62 is controlled by a sum signal.
By a series of these operations, the phase of the reproduction clock and the phase of a clock component of the reproduction digital data can be synchronized with each other and, accordingly, the PRML signal process system becomes applicable, whereby digital data recorded on an optical disk medium can be reproduced stably and accurately.
The conventional optical disk reproducing device, which is constructed as described above, performs demodulation of digital data by a digital signal process in which sampling is performed by an AD converter by employing a clock which is synchronized with a channel bit frequency as a clock component of a reproduction waveform from the optical disk, and the PRML process is performed.
At this time, a PLL circuit, an FIR filter, and a viterbi decoder as constituent elements are processed at a channel bit rate.
However, when digital data demodulation which applies the PRML signal process is performed by employing the reproduction clock which is synchronized with the channel bit frequency of the digital data recorded on the recording medium, the frequency of the reproduction clock is increased at high multiple speed reproduction, that is, at reproduction at a rate higher than a standard reproduction speed for an optical disk, and thus power consumption at a digital circuit is increased dependently on that frequency. Further, the highest reproduction multiple speed is restricted by a bit width in digital operation.
Then, it has been already attempted to perform data demodulation by employing a reproduction clock which is synchronized with a frequency half as high as the channel bit frequency, thereby reducing power consumption at high multiple speed reproduction.
However, in this method, an amount of information relating to a time component after sampling is deteriorated dependently on a reproduction clock of the half frequency, resulting in performance deterioration in the phase locked loop or the transversal filter as described above. Therefore, when there exists local deterioration in reproduction characteristics which depends on a defect generated when the reproduction signal is disturbed due to quality deterioration in the reproduction signal, which depends on the magnitude of a tilt angle that is defined as an angle between an axis perpendicular to the recording surface of the optical disk and an axis of an approaching laser beam which is applied from the reproduction means 51 to the recording surface, or flaws, dirt, fingerprints or the like on the disk surface, it is impossible to maintain the quality of the digital demodulation data and the reading performance in a favorable state. Thus, the above-described method cannot be an effective solution to realize both reduced power consumption and increased reading performance.
The present invention is made to solve the above-described problems and has for its object to provide an optical disk reproducing device which is able to reduce power consumption while maintaining the quality of digital demodulation data and reading performance in a favorable state, even when a signal-noise ratio is deteriorated and a quality of a reproduction signal is locally deteriorated as well dependently on a tilt or a defect.